1. Field of the Invention
The invention relates to gas discharge lasers, particularly to excimer and molecular fluorine lasers having gas mixtures with optimal concentrations of specific component gases, such as halogen containing species, active rare gases, buffer gases, and a xenon additive for improving pulse-to-pulse and peak-to-peak energy stabilities, energy dose stability and burst energy overshoot control, and increasing the lifetimes of laser system components.
2. Discussion of the Related Art
The term xe2x80x9cexcimer laserxe2x80x9d describes gas lasers in which the lasing medium contains excimers (e.g. Ar2*), exciplexes (e.g. ArF*) or trimers (e.g. Kr2F*). The feature common to all is a gas discharge in which highly excited molecules that have no stable ground state are created. The following invention primarily concerns excimer lasers in which the lasing medium is composed of halogen-containing, particularly fluorine-containing exciplexes (e.g. ArF* and KrF*). In addition, the present invention relates to molecular fluorine (F2) lasers.
In a number of scientific, medical and industrial applications for excimer and molecular gas lasers, it is important that the radiation pulses emitted have a stable (constant) energy. In gas lasers, the fact that gas discharge conditions and characteristics can change has an impact on the achievement of a constant energy from pulse to pulse of the emitted radiation. Characteristics and conditions of the gas discharge are dependent upon a number of parameters that with adequate control can allow significant improvements toward exact reproducibility. The result is that the energy of the emitted laser radiation pulses is not maintained exactly constant from pulse to pulse. It is desired to have an excimer or molecular fluorine laser that demonstrates greater pulse-to-pulse stability.
Energy stability is described by various characteristics of the laser beam depending on the application. One of these characteristics is the standard deviation sigma of a distribution of energies of a large number of laser pulses. As many applications use laser output not continuously but in bursts of light pulses, other parameters are also used for stability (see U.S. Pat. No. 5,463,650, which is hereby incorporated by reference into the present application, and particularly the background discussion therein). Specific application of the excimer or molecular fluorine laser beam in optical lithography as an illumination source for wafer scanners, the energy dose stability is significant (see U.S. Pat. No. 5,140,600, which is assigned to the same assignee as the present application, and The Source(trademark) (Cymer, Inc.), Vol. 1, Issue 2 (Summer 1999), each of which is hereby incorporated by reference into the present application).
Another significant characteristic is peak-to-peak stability. For measuring the peak-to-peak energy stability values, laser pulse energies are accumulated over some interval. The absolute difference between the maximum and minimum energies related to the average laser pulse energy is defined as the peak-to-peak stability.
Of particular interest in burst mode applications, the energy overshoot, as illustrated in FIG. 1, is a significant characteristic. Energy overshoot, or spiking, is observed when the laser isoperated with constant high voltage at the discharge chamber in burst mode and the first few pulses have higher energies than pulses later in the burst (see U.S. Pat. Nos. 5,710,787 and 5,463,650, hereby incorporated by reference). The energy overshoot (designated xe2x80x9covsxe2x80x9d in FIG. 1) is defined as the difference between the energy of the first pulse in a burst and the steady state energy in the entire burst.
The quality of the gas discharge and also the pulse energy of the emitted laser radiation pulses are dependent upon and are sensitive to variations in gas discharge conditions such as characteristics of the external electrical circuit, the composition and shape of the gas discharge electrodes, the type and quality of pre-ionization, etc. The purity of the gas mixture in the laser gas discharge chamber and the composition of the gas are also very important. Even tiny impurities of certain kinds are known to be very detrimental to the energy of the emitted radiation pulses, the stability of their energy (the consistency of energy per laser pulse from one firing to the next), the intensity distribution in the laser beam profile, the life of the laser gas and the life of individual optical and other laser components. Such impurities in the gas can be present in the gas mixture from the very beginning or they may form during operation of the laser, e.g. through interactions between reactive components of the laser gas mixture (e.g. of the halogen) and the laser chamber material or through diffusion from the materials or chemical reactions in the gas mixture. For example, during operation of a KrF-excimer laser, such contaminants as HF, CF4, COF2, SiF4 have been observed to increase in concentration rapidly (see G. M. Jurisch et al., Gas Contaminant Effects in Discharge-Excited KrF Lasers, Applied Optics, Vol. 31, No. 12, pp. 1975-1981 (Apr. 20, 1992)). For a static KrF laser gas mixture, i.e., with no discharge running, increases in the concentrations of HF, O2, CO2 and SiF4 have been observed (see Jurisch et al., above).
It is known that the addition of certain substances to the gas mixture can improve particular characteristics of the emitted radiation. For example, U.S. Pat. Nos. 5,307,364 and 5,982,800 (hereby incorporated by reference) suggest that small amounts of oxygen be added to the gas mixture to achieve greater reproducibility of emitted radiation during laser operation. Oxygen, however, is not an inert gas, and its effects on other parameters of the excimer laser, such as the uniformity of the emission intensity curve and the life of the gas mixture are not yet fully understood and may be in fact detrimental. Oxygen, especially atomic oxygen and ozone which can form in the gas discharge, are extremely chemically reactive, and their effects on the laser gas mixture can be quite detrimental, especially during long periods of operation. Due to the presence of oxygen, other stable impurities such as OF2 and FONO form in the excimer laser gas mixture. These can have a considerable absorption effect on the laser irradiation or the pre-ionization radiation. Tests recommended by the current state of technological developments in which the energy of excimer laser radiation impulses is stabilized through the addition of gases to the gas mixture have shown disadvantageous effects on other characteristics of the laser and the emitted radiation.
Filling an excimer or molecular fluorine laser with a gas mixture of precise composition and maintaining that composition is known to be advantageous for determining significant output beam parameters. For example, KrF-excimer laser gas mixtures typically comprise around 1% Kr, 0.1% F2 and a 98.9% Ne buffer. For the ArF-excimer laser, the composition is around 1% Ar, 0.1% F2 and 98.9% buffer. The molecular fluorine laser typically has around 0.1% F2 and 99.9% buffer gas.
The introduction of very small quantities (xe2x89xa70.1 Torr) of xenon in excimer and molecular fluorine laser gas mixtures has been proposed as increasing the photopreionization yield. See R. S. Taylor and K. E. Leopold, Transmission Properties of Spark Preionization Radiation in Rare-Gas Halide Laser Gas Mixes, IEEE Journal of Quantum Electronics, pp. 2195-2207, Vol. 31, No. 12 (December 1995). Taylor et al. demonstrate an enhancement of spark pre-ionization intensity by the action of a Xenon additive to the gas mixture. An advantageous result of this enhancement of the preionization density is an improvement of the homogeneity of the excimer laser discharge. Taylor et al. describe qualitatively, however, that if the xenon concentration is too high, then absorption of laser radiation would occur and degrade the output laser beam. The conclusion of Taylor et al. then is that only a small amount of xenon added to an excimer laser gas mixture would enhance the preionization intensity and improve the discharge.
More recently, the use of xenon in ArF excimer lasers has been reported by Wakabayashi et al. See Wakabayashi et al., Billion Level Durable ArF Excimer Laser with Highly Stable Energy, SPIE""s 24th Annual International Symposium on Microlithography, Santa Clara, May 14-19, 1999. Wakabayashi et al. describe similar results as Taylor et. al (see above), namely, an improvement of the preionization density resulting in an increased output energy at the same input discharge voltage of the ArF-excimer laser. The optimal concentration of xenon in the ArF-excimer laser gas mixture is described as 10 ppm, or the peak of the output energy versus xenon concentration curve shown at FIG. 6 of Wakabayashi et al.
It is recognized in the present invention that an advantageous value of the concentration of an additive, such as a noble gas, e.g., preferably xenon and alternatively krypton to an ArF-excimer laser gas mixture, as well preferably xenon or argon to a KrF-laser, argon or krypton to a XeCl- or XeF-laser, and xenon, argon or krypton to a F2-laser gas mixture, wherein the concentration selected depends not only on its effect on the photo-preionization yield and the output energy, but also on the energy stability and overshoot control of the laser.
It is therefore an object of the invention to provide an excimer or molecular fluorine laser having a gas mixture including an appropriate concentration of the gas additive based at least in part on the effect of the concentration of the additive on improving the energy stability of the output laser beam. The energy stability is determined based on both the stability of the first pulse or first few pulses after a pause for a laser operating in burst mode, and also on the overall stability of the output energy of the laser.
It is a further object of the invention to provide the appropriate concentration of the additive gas based on the effect of the concentration of the additive on improving the overshoot control of the laser.
It is a further object of the invention to provide an excimer or molecular fluorine laser with energy attenuation control to increase the lifetimes of optical and laser tube components.
In accordance with the above objects, an excimer laser, such as a KrF- or ArF-laser, or a molecular fluorine (F2) laser, preferably for high repetition rate operation such as above 1 kHz, is provided with a gas mixture including a small amount of a gas additive. The gas additive is preferably xenon. For the ArF-excimer laser, for stability reasons, the initial concentration of the gas additive is selected and may be adjusted in accordance with selected values of one or more of energy stability, overshoot control, and pulse energy.
The xenon concentration selection may be further based on the additional criteria of output pulse energy control. For example, the pulse energy may be attenuated, e.g., to advantageously lengthen the laser pulses, by decreasing the fluorine concentration in the gas mixture, and then the loss of energy may be compensated by adding an appropriate amount of xenon to the gas mixture. The pulse energy or energy dose may be regulated by controlling the amount of xenon in the gas mixture.
A gas discharge laser such as an excimer or molecular fluorine laser in accord with the present invention includes a laser tube including an electrode chamber containing a pair of elongated main electrodes and one or more prelonization electrodes, and a gas flow vessel. The laser tube is filled with a gas mixture including a laser active gas or gases, a buffer and a trace amount of an additive gas for improvement of burst energy overshoot control and/or a characteristic energy stability such as standard deviation sigma, and/or peak-to-peak, pulse-to-pulse and/or dose stability, and/or adjustment of the output energy level of the laser, such as for energy attenuation control or for balancing the energy stability and/or overshoot control.
The preferred laser system is equipped with an internal gas supply unit including a supply of the additive gas, preferably a xenon supply. An output beam parameter stabilization algorithm is provided for the laser system which maintains optimal concentration of all of the gas mixture constituents including the halogen containing species, F2 or HCl, and the gas additive, preferably xenon, as well as for the active rare gases Ar and Kr for the ArF-laser and the KrF-laser, respectively. Preferred gas control, composition and stabilization algorithms are described at U.S. patent application Ser. No. 09/379,034, Nos. 60/124,785, 60/159,525, Ser. Nos. 09/418,052, 09/317,526 and No. 60/127,062 and U.S. Pat. Nos. 4,393,505 and 4,977,573, each of which is assigned to the same assignee and is hereby incorporated by reference into the present application, wherein the algorithms disclosed in the above patents and patent applications are modified in accord with the present invention to include the injection and control of the gas additive into the gas supply in the discharge chamber. Such parameters as energy stability, energy dose stability, output pulse energy and driving voltage (and/or amplified spontaneous emission (ASE) and/or features of the temporal or spatial pulse shape and/or one or more other parameters such as total accumulated energy input to the discharge, bandwidth, moving average energy dose, temporal or spatial coherence, discharge width, and long and short axial beam profiles and divergences, time, pulse count or a combination thereof) may be monitored and parameters of the output beam mentioned above and/or others are stabilized in accord with the present invention.
The control of the amount of the gas additive in the gas mixture is also preferably used to increase the lifetimes of laser components. The characteristic output power range is initially set to be higher than the desired output power of the laser system, within the range of operating driving voltages. Then, the power is attenuated by adding more of the gas additive, preferably xenon, into the gas mixture until the output power is reduced to the desired level. As the laser components age, the amount of additive/xenon is reduced to achieve the desired output power with each new fill.
The gas additive may be added to the gas mixture from a gas container including a premix including the preferred xenon gas additive. Alternatively, xenon gas can be obtained from xenon containing crystals that are heated to dissociate the xenon containing crystals. In this embodiment, a xenon generator is filled with xenon-containing crystals and a heating element and temperature controller are used to control the xenon gas pressure.
Although xenon is the preferred gas additive, other gas additives may be used in accord with the present invention. Argon may be used as the gas additive for a KrF laser. Krypton may be used as the gas additive for an ArF laser. Argon and/or krypton may be used as the gas additive for a XeCl or XeF laser. Argon, Krypton and/or Xenon may be used for a F2 laser. NO may be used for a XeCl laser (e.g., 0.1% NO in Ne). NO2, N2O4, FONO or FNO may be used for a XeCl or F2 laser.
Another element or molecule, such as a metal, e.g., W or Pt, may be added that would react to form one or more metal fluoride or metal chloride species, i.e., preferably WF, WF2, PtF, PtF2 or alternatively WFx or PtFx, wherein x is preferably between three and sixteen, within the gas mixture. The metals may be added to one or more electrodes preferably of the preionization unit or another metal component of the laser tube, if any. Other candidate metals include chromium, and aluminum. Silicon, carbon, hydrogen fluoride, ozone, mercury, hafnium, metals and alloys having high vapor pressure similar to mercury and hafnium, such as are typically liquids at standard temperature and pressure (STP) may be used. Some metal oxides such as molecular combinations of oxygen and one or more of chromium, fluorine or aluminum, are other preferred candidate elements or molecular species that may be used and/or that are or will form halides (i.e., fluorides or chlorides), may be used as the gas additive, wherein xenon is herein described as being preferred.
Some particular preferred molecular combinations, either neutral or ionized or combinations of neutral and ionized species, that may be added or that may be formed by an additive reacting with the fluorine or chlorine already in the gas mixture include HF, HF, CFx (particularly CF4), CrOF2, CrOF, CrO2F, CrO2F2, CrO2, CrO, Cr, CrF2, CrF, SiF4, SiF, OF, O2F, OF2, Al, AlO, Al2O, Al2O2, AlF, and AlF2. Other possibilities include N, N2, Nx, C, C2, Cx, H, H2, Hx, O, Ox where x is a small integer above 3, such as 3-16, and combinations of any of these elements and/or molecules, as well as air itself. Any of the above mentioned elements or molecules or combinations thereof may be added to the gas mixture, preferably in trace amounts such as less than 500-1000 ppm, or less than 0.1%, in accord with the present invention.
In addition, more than one gas additive may be added to the gas mixture. For example, two or more of the additive mentioned above may be added to the gas mixture for controlling the pulse energy, energy dose, energy stability and/or overshoot control, either separately or in combination. One gas additive, or combination of gas additives, may be used to control one of these parameters or others, and another gas additive, or combination of gas additives, may be used to control another of the above parameters.